Background remnants

I am working with Captivate 3. I've replaced backgrounds in some slides with
ones that I captured in another movie (same resolution) and imported. This
works great in most places. But in a couple of slides ... when I preview, I
see a remnant of the old background at the very end of the slide, for a
fraction of a second. I've cleaned out all unused items in the library ...
saved, closed and reopened the file. It's still there. This only happens in
a handful of slides ... but it happens in a couple of my files.

Any idea what could be causing this? And more importantly ... any
suggestions on what I can do to fix it?

Thanks for any help.

Marge